Atomization is a process by which a liquid is dispersed into very fine droplets. The droplets in an atomized liquid are often less than 200 microns in diameter and can be as small as about 10 microns. Atomized liquids are used in many applications including, for example, fire-suppression, fuel-combustion, coating processes, pharmaceuticals, and metallurgy, to name but a few.
Atomized liquid is generated using an atomizer. A variety of atomizer designs exist. One common type of atomizer is the “Hartman” atomizer. In a Hartman-type atomizer, a high-velocity (supersonic) gas stream impinges on a cavity resonator. The resonator abruptly brakes the supersonic gas stream, which results in the creation of shock waves. A stream of liquid exits the atomizer in the vicinity of the shock waves. The energy in the shock waves atomizes the liquid. Examples of Hartman atomizers include the atomizers disclosed in U.S. Pat. Nos. 6,390,203 and 4,408,719. The atomizer that is disclosed in U.S. Pat. No. 6,390,203, which was developed by one of the present inventors, is discussed below.
The atomizer disclosed in U.S. Pat. No. 6,390,203 is reproduced in FIG. 1 as atomizer 100. That atomizer includes rod 102, inner casing 104, outer casing 110, and head 116. Annular gas feed channel 106 is defined between rod 102 and inner casing 104. The gas feed channel leads to annular gas nozzle 108. Annular liquid feed channel 112 is defined between inner casing 104 and outer casing 110. The liquid feed channel leads to annular liquid nozzle 114. Resonator 118 is defined as an annular channel within head 116. The resonator is spaced apart from and situated in opposition to gas nozzle 108.
In operation, a subsonic flow of gas (e.g., nitrogen, etc.) is directed to gas feed channel 106. Gas is discharged from gas nozzle 108 at the speed of sound. Once discharged, the gas expands and its speed becomes supersonic. The gas is abruptly decelerated by resonator 118, which causes acoustic oscillations (i.e., shock waves) in atomization zone 120. The oscillations cause liquid (e.g., water, etc.) that is delivered to atomization zone 120 through liquid nozzle 114 to atomize. A mist of water droplets exits atomizer 100 through ring-shaped outlet 122.
In a Hartman atomizer, the amount of liquid that is atomized is proportional to the amount of shock waves produced. It is convenient, then, to express the efficiency of a Hartman atomizer in terms of the amount of shock waves that are produced by a given volume of gas (passing through the atomizer). To calculate the efficiency (according to this definition), the power, Pgj, (i.e., energy per time) of the gas jet issuing from the nozzle is calculated. This calculation is readily performed knowing the rate of gas discharge and its density. Shockwave power, Psh, is measured in known fashion and the percentage efficiency of the atomizer is calculated by obtaining the ratio of the power in the shockwave to the power of the gas jet and then multiplying that ratio by one hundred:Eff=(Psh/Pgj)×100  [1]
The efficiency of standard Hartman atomizers, such those described in U.S. Pat. Nos. 6,390,203 and 4,408,719, is usually relatively low, being in a range of about five to eight percent.
The prior-art includes alternatives to Hartman-type atomizers, but these other atomizers typically exhibit even lower efficiency than Hartman atomizers. For example, U.S. Pat. No. 4,205,788 discloses a “swirl” atomizer. In this type of atomizer, a swirl chamber imparts rotary motion to a gas. The swirling gas is passed through a nozzle, which intensifies the degree of swirling and generates some acoustic oscillations, which atomize a liquid. The swirling gas contains relatively little energy and these atomizers operate at a very low efficiency of about 0.5 to about 1.0 percent.
In another type of atomizer, liquid is atomized via a substantially stationary decrease in compression. In this type of atomizer, as exemplified by U.S. Pat. No. 5,495,893, bubbles of pressurized gas are dispersed in a liquid. The gas-liquid mixture is then exposed to a substantially instantaneous reduction in pressure (such as is caused by a sudden, large increase in flow area). (See also, U.S. Pat. No. 6,142,457.) The reduction in pressure causes the gas bubbles to rapidly expand and atomize the liquid. The mixture is then accelerated to supersonic velocity through a nozzle. As the mixture decelerates to sonic velocity, shock waves are produced, which further decrease the size of the droplets in the atomized liquid.
The efficiency of “stationary-decrease-in-compression” atomizers is typically within a range of about 2 to 3 percent. The reason for the low efficiency is that these atomizers produce relatively few shock waves per unit time, since oscillation does not occur as in a Hartman atomizer.
In circumstances in which an unlimited amount of gas and liquid is available for use in an atomizer, the benefits of a higher-efficiency atomizer are not immediately clear. But in circumstances in which gas and liquid availability is severely limited, the benefits of increased efficiency are manifest. An example of an application in which these resources are strictly limited—and in which atomizer efficiency is therefore very important—is fire-suppression in aircraft.
Many existing fire-suppression systems for aircraft use a fluorine-containing material (e.g., Halon®). This material has been associated with the depletion of the ozone layer and has been banned by the international community for general use. Aircraft are, however, exempt from this ban and are allowed to continue to use Halon®-based fire-suppression systems until a viable alternative is developed. One potential alternative to Halon®-based systems is a system that uses an atomizer to generate a water mist. The water mist, along with a quantity of nitrogen gas that atomizes water to create the mist, is discharged to suppress a fire. (See, e.g., U.S. Pat. No. 6,390,203.) There are strict weight allowances on aircraft, and nitrogen and water are not exempt from them. As a consequence, it is critically important that a nitrogen/water mist fire-suppression system includes a relatively more-efficient atomizer, which will use less nitrogen (thereby saving weight) to provide a given quantity of water mist than a relatively less-efficient atomizer.
Notwithstanding the foregoing, there has been little progress made toward improving the efficiency of atomizers. It might be supposed that since atomizers have such a relatively uncomplicated structure, little can be done to improve their efficiency. Or, in view the relatively sophisticated understanding of the fluid dynamics of gas flow and the production of shock waves that prevails in the art, it might be supposed that all that can be done to improve atomizers has been done. These suppositions would, however, be incorrect.
While simple in outward appearance, an atomizer, such as a Hartman atomizer, is extraordinarily complex in terms of the fluid dynamic and acoustic behaviors that govern its operation. And to the extent that these behaviors are understood, the prior art has demonstrated little ability to apply this understanding to the development of higher-efficiency atomizers. But it is one thing to understand the theories, it is quite another to apply them to develop a specific atomizer configuration that exhibits improved efficiency. A better explanation for any lack of progress toward the development of higher-efficiency atomizers is simply the complexity of the problem. Notwithstanding sophisticated modeling techniques, this problem is so complex that improvements are at least as likely to come from empirical studies and observation as from theoretical consideration of the problem.